The present invention relates to a switching power supply of the voltage resonance type and, more particularly, to an apparatus which performs the switching operation due to self-oscillation.
Switching power supplies have such features as being small in size, light-weight and have high efficiency therefore, they are widely used as power sources for a number of electronic apparatuses such as electronic apparatuses for use in industry, electronic appartuses for use at home or the like.
One such kind of switching power supply is a voltage resonance type apparatus disclosed in U.S. Pat. No. 4,318,164. When considering a case where a self-oscillation system is applied to such a voltage resonance type switching power supply, in general, a blocking oscillator is applied and the power supply is constituted in a manner as shown in FIG. 1.
Practically speaking, a primary winding 1a of a power transmitting transformer 1 is connected to an input DC power source 3 in series through the collector-emitter of a transistor 2 serving as a switching element. A parallel circuit consisting of a resonance capacitor 4 and a damper diode 5 is connected between the collector-emitter of the transistor 2. A time constant circuit consisting of a series circuit of a resistor 6 and a capacitor 7 is connected in parallel to the power source 3. The connecting point of the resistor 6 and capacitor 7 of the time constant circuit is connected to a base of the transistor 2 through a tertiary winding 1c of the transformer 1. The transistor 2 is controlled by a control signal which is applied to this base. A predetermined AC voltage is developed in a secondary winding 1b of the transformer 1 due to the self-oscillating action which is caused by the operation of the transistor 2 being positively fed back through the tertiary winding 1c and the voltage resonance action by the capacitor 4 in the switching operation, due to this self-oscillation. The voltage developed in the secondary winding 1b of the transformer 1 is outputted as the DC voltage through a rectifying/smoothing circuit consisting of a diode 8 and a capacitor 9.
The fundamental operation of this apparatus is that the voltage of the primary winding 1a of the transformer 1 becomes a waveform as shown in FIG. 2A, the voltage between the collector-emitter of the transistor 2 becomes a waveform as shown in FIG. 2B, and the collector current of the transistor 2 becomes a waveform as shown in FIG. 2C.
However, it is ideal that the voltage resonance switching section, which is constituted by the portions of the winding 1a, resistor 6 and capacitor 7 excluded from FIG. 1, operates to provide the waveforms as shown by broken lines in FIGS. 2A to 2C.
Namely, in such an arrangement, where it is intended to cause voltage resonance while allowing the self-oscillation to be performed, a certain time difference is needed in principle between the time when the transistor 2 is turned on and the time when the voltage of the primary winding 1a of the transformer 1 is inverted. A predetermined time is necessary until the voltage between the emitter-collector of the transistor 2 becomes zero after the terminal voltage of the primary winding 1a of the transformer 1 was inverted. If the transistor 2 is turned on within this interval, the resonance capacitor 4 is short-circuited by the transistor 2, and the charges stored in the capacitor 4 are discharged through the transistor 2. In the arrangement shown in FIG. 1, since the terminal voltage of the tertiary winding 1c of the transformer 1 is inverted instantaneously when the terminal voltage of the primary winding 1a of the transformer 1 is inverted, it is impossible to provide the foregoing proper time difference. Thus, a remarkable increase in electric power loss cannot be avoided in the arrangement shown in FIG. 1.
As described above, in the switching power supply which transmits power with high efficiency due to the switching operation using the voltage resonance, it is very difficult to control the switching operation of the main switching element by way of the self-oscillation while maintaining the voltage resonance condition.